Anaerobic microorganisms can produce alcohols and other products from carbon monoxide (CO) through fermentation of gaseous substrates. Fermentations using anaerobic microorganisms from the genus Clostridium produce ethanol and other useful products. For example, U.S. Pat. No. 5,173,429 describes Clostridium ljungdahlii ATCC No. 49587, an anaerobic microorganism that produces ethanol and acetate from synthesis gas. U.S. Pat. No. 5,807,722 describes a method and apparatus for converting waste gases into organic acids and alcohols using Clostridium ljungdahlii ATCC No. 55380. U.S. Pat. No. 6,136,577 describes a method and apparatus for converting waste gases into ethanol using Clostridium ljungdahlii ATCC No. 55988 and 55989.
A gaseous substrate in the form of syngas may provide CO to the fermentation. Syngas may include CO, CO2 and H2. However, relative amounts of CO, CO2 and H2 in the syngas may vary depending on how the syngas is generated. Some sources of syngas may include elevated levels of CO2. Optimal conversions of CO into alcohols and other products and enhanced utilization of CO2 may be dependent on relative levels of CO, CO2 and H2 in the syngas provided to the fermentation.